1. Field of the Invention
Example embodiments of the present invention relate to an ohmic layer, a method of forming an ohmic layer, a semiconductor device including an ohmic layer and a method of forming a semiconductor device including an ohmic layer. More particularly, example embodiments of the present invention relate to an ohmic layer reducing a resistance between silicon and metal, which may be used in a semiconductor device.
2. Description of the Related Art
As semiconductor devices have become highly integrated, sizes of patterns of semiconductor devices have been decreased and intervals among the patterns have been reduced. When conductive patterns, for example, gate electrodes and/or bit lines have reduced sizes, resistances of the conductive patterns may increase.
Polysilicon doped with impurities may be used for a conventional conductive pattern such as a gate electrode. However, a conductive pattern of doped polysilicon may not be employed in a highly integrated semiconductor device having a high response speed because the reduced size of the conductive pattern of doped polysilicon may cause the conductive pattern of doped polysilicon to have a relatively high resistance.
Therefore, a polycide structure was developed for forming a conductive pattern because the conductive pattern of a polycide structure may have a resistance lower than that of a conductive pattern of doped polysilicon. Conventionally, a polycide structure may include a doped polysilicon film and a refractory metal silicide film, for example, a tungsten silicide film formed on a doped polysilicon film. However, a conductive pattern of a polycide structure may not be employed in a conventional semiconductor device requiring an exceedingly low resistance, although the conductive pattern of the polycide structure has the low resistance.
Considering the above-mentioned problem, a conductive pattern including a polysilicon film and a metal film has been developed for a conventional semiconductor device. However, a metal film of a conductive pattern may be easily silicided in a formation of the metal film on a polysilicon film, so a barrier layer may be formed between the polysilicon film and the metal film in order to reduce a silicidation reaction between the polysilicon film and the metal film. Further, an ohmic layer may be formed between a polysilicon film and a metal film to reduce a total resistance of a conductive pattern.
An ohmic layer is generally formed between a metal wiring and a silicon layer or a silicon substrate when a metal wiring is formed on a substrate. For example, a prior art method of forming a gate electrode may include a polysilicon layer pattern and a tungsten layer pattern. In particular, a prior art gate electrode may include a polysilicon layer pattern, a tungsten silicide layer pattern, a diffusion barrier layer pattern and the tungsten layer pattern. A tungsten silicide layer pattern of the prior art gate electrode may include tungsten and silicon with an atomic ratio in a range of about 1:2 to about 1:3.
When a tungsten silicide layer pattern having tungsten and silicon by the atomic ratio in the range of about 1:2 to about 1:3 is formed between a polysilicon layer pattern and a tungsten layer pattern, however, ingredients in a diffusion barrier layer pattern may be easily decomposed in a succeeding thermal process so that the tungsten layer pattern may be easily silicided, and then the tungsten layer pattern may be converted into a silicide layer pattern. When a tungsten layer pattern is converted into a silicide layer pattern, a gate electrode may not have a desired low resistance. Additionally, a volume of a tungsten layer pattern is reduced when the tungsten silicide layer pattern is silicided so that a void may be generated at an interface between a polysilicon layer pattern and a tungsten silicide layer pattern.